After rough idling took my 07 XC70 with 240k km to dealer where a compression test revealed major compression lost in cylinder 4 (40 vs all others at 145).

When pulling the head they discovered a burnt exhaust valve with a chip on the edge, stating this was the problem. Engine shop wanted $5700 to rebuild head replacing all valves while new head with guts quote was $6200. I decided to go for the new as I plan to keep the vehicle. I've been using 91 grade fuel, regular service by dealer and overall babying the car inside and out.

What would cause this issue? How to avoid in the future, has anyone else had a similar issue and with what results? Oddly enough a colleague with a 2004 XC70 with 280k km had the same issue with two burnt and chipped valves. He had only the 2 replaced at a cost of $4000. Dealer says it hardly ever happens, luck of the draw? Should i use a fuel additive for a cleaner burn as the valve stem had carbon buildup as well? Feedback would be appreciated.